![]() ![]() In a 2012 study, researchers analyzed the brains of 59 women ages 32 to 101, after they had died. In some cases, however, the fetal cells may stay in a woman's body for years. ![]() In some cases, the fetal cells remain in the mother’s organs for years. A 2015 study suggested that this happens in almost all pregnant women, at least temporarily. Because the cells are from a different individual, the mother will have two sets of DNA at least for a while. During pregnancy, a small number of cells from the fetus migrate into the mother's blood and travel to different organs. There is also a kind of chimerism, called microchimerism, that particularly affects pregnant women. This may be called “complete donor chimerism”. Because bone marrow contains stem cells that develop into red blood cells, a person with a bone marrow transplant will have blood cells that are genetically identical to those of the donor while other types of cells in their own bodies may remain the same, turning the recipient into a human chimera. This is called fusion chimerism, or tetragametic chimerism.Ĭhimerism in humans can also happen as a result of a bone marrow transplant, for example, to treat leukemia or diseases like aplastic anemia, and sickle cell anemia. Human chimeras may occur naturally when two nonidentical twins merge in the womb early in development. If the early embryos fuse together, so that the cells of one are absorbed by the other in the early stages of development, they can become a single chimera. Most human chimeras do not have any observable symptoms - but may have an increased risk of autoimmune disease. However, they may have different-colored eyes, patchy skin, differential hair growth, two different types of red blood cells, intersexuality, or genital ambiguity, but this is not very common. Humans who are chimeras usually don't know it. In a mosaic, all the cells originate from the same original organism and the genetic changes were caused by genetic mutations.Ĭhimerism is very rare, except in cats, where it may actually be fairly common. However, a condition where two or more populations of genetically distinct cells emerge in the same zygote is called a mosaic, rather than a chimera. Source: Gataquimera/Instagram & Raquel Baranow/Wikimedia Commons For example, the animal can end up with one organ made up entirely of cells of one genotype, and another organ completely built from cells of the other genotype. During development, there is a chance that entire organs will possess different genotypes. Each population of cells keeps its own genotype and its own characters. As a result, an animal chimera will carry two or more genetically distinct populations of cells, originating from different zygotes. In animals, chimeras can occur after either two fertilized eggs or two early embryos fuse together into one organism. In genetics, chimerism is a biological condition and has nothing to do with monsters. ![]() In Greek mythology, it was used to describe a fire-breathing hybrid creature composed of different animals, that was often depicted as a lion with a goat’s head on its back and a tail that ends with a snake's head. More multidisciplinary research is required for a better understanding of this fascinating subject.The word “chimera” comes from the ancient Greek, meaning 'she-goat' or 'monster'. If human chimeras are more common than hitherto thought there could be many medical, social, forensic, and legal implications. To date there are no examples in humans of twin chimeras involving germ cells. In marmoset monkey twins the exchange via the placenta is not limited to blood but can involve other tissues, including germ cells. Blood chimeras are formed by blood transfusion between dizygotic twins via the shared placenta and are more common than was once assumed. Only 28 of the 50 individuals with a 46,XX/46,XY karyotype were either true hermaphrodites or had ambiguous genitalia. Even sex-discordant chimeras can have a normal male or female phenotype. Many are discovered accidently, for example, during a routine blood group test. Most chimeras remain undetected, especially if both zygotes are of the same genetic sex. Two zygotes can fuse together during an early embryonic stage to form a fusion chimera. Fetal and maternal cells can cross the placental barrier so that both mother and child may become microchimeras. Natural chimeras can arise in various ways. ![]() The first human chimera was reported in 1953. This review concerns natural human chimeras. Recipients of tissue and organ transplants are artificial chimeras. A chimera is an organism whose cells are derived from two or more zygotes. The term chimera has been borrowed from Greek mythology and has a long history of use in biology and genetics. ![]()
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